The photosystem I mutant viridis‐zb63 of barley (Hordeum vulgare) contains low amounts of active but unstable photosystem I

Abstract

The barley mutant viridis‐zb63 is a nuclear gene mutant that was previously reported to be completely devoid of photosystem (PS) I activity. This investigation was carried out in order to determine the activity level with more sensitive techniques. Flash‐induced absorbance changes and photoreduction of NADP+ showed that the mutant does in fact exhibit about 2% PSI activity compared to the wild‐type, but the mutant has a less stable PSI. A second objective was to determine if the deficiency in PSI was due to the absence of a specific subunit. The mutant contains PSI transcripts in the correct sizes and in similar amounts as in the wild‐type. Isolated chloroplasts of the mutant synthesize the core polypeptides PSI‐A and PSI‐B as well as the iron‐sulfur protein PSI‐C and are capable of import, processing and integration of nuclear‐encoded PSI precursors into the thylakoids. By western blotting, all investigated PSI polypeptides are detected in the mutant thylakoids at a level of 1 to 30% of wild‐type amounts. One polypeptide, PSI‐G, occurs at significantly higher level than the other PSI polypeptides and is possibly associated with the light‐harvesting complex of PSI in the mutant thylakoids. Since the results obtained document that each of the structural components of the PSI complex appears normal in the mutant viridis‐zb63, we conclude that the low level of PSI activity most likely reflects the lack of a factor important for translation of PSI‐A/B or a protein factor that is necessary for assembly of the PSI complex, but is not a component of the functional complex.